These systems exhibit three-phase induction or more. Three phase power is commonly found in industrial applications. Normal household supply consists of a sine wave of alternating polarity, with the voltage rising and falling over time. This is called single phase electric power.
A few older installations in the U.S. used two-phase four-wire systems for motors. The chief advantage of these was that the winding configuration was the same as for a single-phase capacitor-start motor. Most of these have been upgraded to three-phase systems. A two-phase supply with 90 degrees between phases can be derived from a three-phase system using transformers in a Scott connection.
Polyphase power is particularly useful in AC motors, where it can be used to generate a rotating magnetic field. Alternate current sine wave voltage points, when the field is graphed and analyzed, show a result of a magnetic field that is rotating (thus producing the alternating polarity). When a three-phase voltage completes one full cycle, the magnetic field has rotated through 360°. In a polyphase system several such supplies are overlapped out of phase to smooth out the total voltage.
Nikola Tesla invented these types of systems. Polyphase electrical systems operate on the principle of a alternate-current circuits' rotating magnetic field lead-lag effect. He found these systems in his research of harmonic oscillators